I notice one of my zebra pleco is passing out long clear string of poo with bloated stomach. I suspect that it might have internal parasite. Can anyone recommend what medication is safe for pleco and should I treat the whole tank or just the affected fish?

Its is hard to give a good answer without more information. For one, what medications can be found varies from country to country. Because the is no location information supplied by ystan, it is impossible to suggest a med.

Next, a bit more information would be helpful. I have a variety of medications I use for internal digestive issues. These can be caused by a variety of different things. It would help to know more information here to include:

1. How long have you have the fish with the problem?
2. Is is a wild caught or a tank raised fish?
3. Is the fish still eating? (This matters in terms of how various meds are administered.)
4. What diet do you normally feed?
5. Are there other fish in this tank and if so do any of them appear to have similar symptoms?

Metro works best when fed. If your fish is not eating, a metro bath can be tried. I have several meds as I mentioned, Some work well in the water while others do not. Meds I have internal parasites or worms include:
Metrodinazole
Fluebendazole
Lavamisole HCL
Praziquantel (PraziPro)

It is also possible in the states to find medicated foods. I have metro infused food as well as anti-biotic infused food as well as the individual drugs.

The problem with fish is any disease which does not exhibit clear symptoms which point to something specific, we are mostly working in the dark.

Thanks for the reply.
I notice it for about a week. Its a wild caught fish ,currently only 1 is having this problem out of a batch of 10 L46 and 2 L600. The fish is still feeding and I am feeding it sera viformo, sera spirulina, hikari carnivore pellet and Dr bassleer garlic. no sunken stomach is notice.

seachem paraguard, Eiho prazi gold and hikari prazi pro is available here but not sure if they are safe? Should I medicate the whole tank or just the affected fish and is it ok to use Praziquantel as preventive measure?

Metrodinazole would be best if you can find it. Feeding it works best so if the fish are not eating, I would go with one of the other two above. In the states I can find medicated foods containing Metro. Since I still don't know where you are, so I cannot offer help on this overseas.

Parasites can often be spread from fish to fish, so I would treat the whole tank.

I tend not to mix meds. While some may be fine together, others may not. As a rule I do not want to use two different medications to treat the same problem at the same time. I will use the one that is best suited to the situation. This may be as simple as if the med can be fed it if has to go in the water. Most medications are usually administered for about 5 - 7 days. This can be a daily dose or it may be spaced differently.

I have found that the need to do a second round of medication is more common with antibiotics. Some things need a second round, sometime with a larger dose, to fix. Bear in mind that most medications even as they are working on an underlying problem are also somewhat stressful for the fish.